Electrochemical Tongue

ABSTRACT

An electrochemical tongue can be used for detection of metal ions. The reference electrode of the electrochemical tongue can be coated with a polymer. More than one reference electrode can be used, and the electrochemical tongue can be inserted into a cone penetrometer for portable, in situ analysis.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/808,102, filed on Apr. 3, 2013. The entire teachings of whichapplication are incorporated herein by reference. The entire teachingsof the above application(s) are incorporated herein by reference.

GOVERNMENT SUPPORT

This invention was made with Government support under NSF-CMMI-1031505from the National Science Foundation. The Government has certain rightsin the invention.

BACKGROUND OF THE INVENTION

Known methods used for the detection of trace concentrations of metalion contaminants generally utilize sophisticated analytical tools suchas atomic absorption spectroscopy (AAS) and inductively coupled plasmamass spectrometry (ICPMS). These techniques, while extremely sensitive,require expensive instrumentation and highly trained personnel. Inaddition, they are frequently non-portable, which severely limits theirsuitability for on-site detection of metal ions. Test solutions must besampled at the contamination site and then shipped off-site to a lab,where they are tested using the techniques mentioned above. For thesereasons, the tests are expensive and can potentially expose laboratorypersonnel to hazardous materials.

Highly sensitive chemical sensing systems integrated within direct pushtechnologies (DPTs), such as a cone penetrometer (CPT) or membraneinterface probe (MIP), have become valuable tools for contaminantcharacterization in complex soil matrices. By providing the possibilityfor rapid, real-time, on-site analysis, a CPT instrumented with sensorscan serve to ensure the safety of laboratory personnel and field workersby minimizing exposure to contaminated soil samples. In recent years, adeviation from these techniques has been presented through a multitudeof sensors and sensing systems integrated within DPTs, but issues ofsimplistic system design, cost, functionality and efficiency have yet tobe overcome. Existing systems are often relatively complex, do notaccommodate both qualitative and quantitative analysis of inorganicspecies, and require highly trained operators. In particular, thesesystems may not detect metal ions at a sufficiently low concentration.In other words, the sensitivity is insufficient.

Thus, there is a need in the art for a system that can detect metal ionsat a lower level of detection. Preferably, such a system would beportable, in order to provide qualitative and quantitative in situdetermination of the metal contaminants in groundwater and saturatedsoil.

SUMMARY OF THE INVENTION

Disclosed herein is an electrochemical tongue. The electrochemicaltongue can include a reference electrode, a counter electrode, one ormore working electrodes, wherein at least one of the one or more workingelectrodes is coated with a polymer or copolymer having the formula (I):

X can be independently NH or O; r can be independently an integer fromapproximately 1 to approximately 15; and Y can be chelating agent. Theelectrochemical tongue can also include a potentiostat in electricalcommunication with the reference electrode, the counter electrode, andthe one or more working electrodes. The electrochemical tongue can alsohave at least two working electrodes that are of distinct materials. Oneor more working electrodes can be formed from gold, carbon fiber,silver, platinum, and transparent conductive oxides. At least one of theone or more working electrodes can be of glassy carbon, carbon paste,carbon fiber, carbon nanotubes, and graphene. One of the one or moreworking electrodes can include conductive metal oxides coated on rigidor flexible substrates. At least one of the working electrodes can becoated with a polymer having the formula (II):

X can be NH or O; r can be an integer from approximately 1 toapproximately 15; n can be an integer from approximately 6 toapproximately 100; and Y can be a chelating agent. At least one of theat least two working electrodes is coated with a polymer having theformula (III):

X can be NH or O; r can be a number of methylene groups between 1 and15; m can be an integer from approximately 6 to approximately 100; n canbe an integer from approximately 6 to approximately 100; and Y can be achelating agent. Preferably, the sum of m+n is at least 10. Thechelating agent Y can be an aminocarboxylic acid; a hydrocarboxylicacid; ethelene diamine; diethylenetriamine; triethylenetetramine;triaminotriethylamine; polyethyleneimine; triethanolamine;n-hydroxyethylethylene diamine; 2-aminopyridine; 4-aminopyridine; 2,2′dipicolylamine; 5,6 diamino-1,10 phenanthroline; thioglycolic acid;gluthathione; or diethyl dithiophosphoric acid. In particular, thechelating agent Y is an aminocarboxylic acid. The aminocarboxylic acidis an iminodiacetic acid or n-hydroxyethyl glycine. The chelating agentY is a hydroxycarboxylic acid. The hydroxycarboxylic acid can betartaric acid, citric acid, or gluconic acid. The electrochemical tonguecan further include one or more of a voltammetric sensor, anamperometric sensor, and a potentiometric sensor. The voltammetricsensor includes one or more of a linear sweep sensor, a cyclic sensor, astair case sensor, a differential pulse sensor, a square wave sensor,and an anodic/cathodic stripping voltammetry sensor. The electrochemicaltongue can further include a potentiometric sensor. The electrochemicaltongue can further include one or more of an redox sensor, a pH sensor,an electrical conductivity/sensitivity sensor, a dissolved oxygensensor, and a selective ion selective sensor. The electrochemical tonguecan further include a housing defining a passageway for the referenceelectrode, counter electrode, and one or more working electrodes. Thehousing can be adapted for insertion into a penetrometer. Theelectrochemical tongue can further include a porous portion that allowsa sample to enter the electrochemical tongue. At least a portion of atleast one of the one or more working electrodes can include sensingsurfaces that have been modified by at least one of gel integration andbismuth/mercury coating. The one or more of the working electrodes canhave an ionically conductive fluoropolymer overcoating. Theelectrochemical tongue can further include a processor programmed toreceive a voltammetric response, filter and extract features, build adecision tree and a linear model, and display the identity of a metalion.

Disclosed herein is also a method for performing voltammetry. The methodincludes contacting a sample to be analyzed with an electrochemicaltongue, applying a constant voltage across the one or more of theworking electrodes to reduce the metal ion onto the surface of theelectrode, and increasing the voltage across one or more workingelectrode to oxidize and strip off the metal from the surface of theelectrode. The electrical tongue can include a reference electrode, acounter electrode, one or more working electrodes, and a potentiostat inelectrical communication with the reference electrode, the counterelectrode, and the one or more working electrodes. At least one of theone or more working electrodes can be coated with a polymer or copolymerhaving the following formula (I):

The variable X can be independently NH or O; r can be independently aninteger from approximately 1 to approximately 15; and Y can be achelating agent. The electrochemical tongue can have at least twoworking electrodes of distinct materials to which the sample iscontacted. At least one of the one or more working electrodes can befrom gold, carbon fiber, silver, platinum, and transparent conductiveoxides. At least one of the one or more working electrodes to which asample is contacted can be of glassy carbon, carbon paste, carbon fiber,carbon nanotubes, or graphene. At least one of the working electrodes towhich a sample is contacted can include conductive metal oxides coatedon rigid or flexible substrates. At least one of the at least twoworking electrodes to which a sample is contacted is coated with apolymer having the formula (II):

The variable X can be NH or O; r can be an integer from approximately 1to approximately 15; n can be an integer from approximately 6 toapproximately 100; and Y can be a chelating agent. At least one of theat least two working electrodes to which a sample is contacted can becoated with a polymer having the formula (III):

The variable X can be NH or O; r can be an integer from approximately 1to approximately 15; m can be an integer from approximately 6 toapproximately 100; n can be an integer from approximately 6 toapproximately 100; and Y can be a chelating agent. Preferably, the sumof m+n is at least 10. The chelating agent Y coating the one or moreworking electrodes to which a sample is in contact can be anaminocarboxylic acid; a hydrocarboxylic acid; ethelene diamine;diethylenetriamine; triethylenetetramine; triaminotriethylamine;polyethyleneimine; triethanolamine; n-hydroxyethylethylene diamine;2-aminopyridine; 4-aminopyridine; 2,2′ dipicolylamine; 5,6 diamino-1,10phenanthroline; thioglycolic acid; gluthathione; or diethyldithiophosphoric acid. In particular, the chelating agent Y can be anaminocarboxylic acid. The aminocarboxylic acid can be an iminodiaceticacid or n-hydroxyethyl glycine. The chelating agent Y can be ahydroxycarboxylic acid. The hydroxycarboxylic acid can be tartaric acid,citric acid, or gluconic acid. The method can further include measuringa first sampling current that flows through the one or more workingelectrodes during a predetermined interval in which the pulse voltage isnot applied, measuring a second sampling current that flows through theone or more working electrodes while the pulse voltage is applied, andcalculating the difference between the first and second samplingcurrents.

Disclosed herein is also a method for performing voltammetry. The methodincludes contacting a sample to be analyzed with an electrochemicaltongue, and ramping the working electrode voltage linearly versus timeto either positive or negative voltages. The electrical tongue caninclude a reference electrode, a counter electrode, one or more workingelectrodes, and a potentiostat in electrical communication with thereference electrode, the counter electrode, and the one or more workingelectrodes. At least one of the one or more working electrodes can becoated with a polymer or copolymer having the following formula (I):

The variable X can be independently NH or O; r can be independently aninteger from approximately 1 to approximately 15; and Y can be achelating agent. The electrochemical tongue can have at least twoworking electrodes of distinct materials to which the sample iscontacted. At least one of the one or more working electrodes can befrom gold, carbon fiber, silver, platinum, and transparent conductiveoxides. At least one of the one or more working electrodes to which asample is contacted can be of glassy carbon, carbon paste, carbon fiber,carbon nanotubes, or graphene. At least one of the working electrodes towhich a sample is contacted can include conductive metal oxides coatedon rigid or flexible substrates. At least one of the at least twoworking electrodes to which a sample is contacted is coated with apolymer having the formula (II):

The variable X can be NH or O; r can be an integer from approximately 1to approximately 15; n can be an integer from approximately 6 toapproximately 100; and Y can be a chelating agent. At least one of theat least two working electrodes to which a sample is contacted can becoated with a polymer having the formula (III):

The variable X can be NH or O; r can be an integer from approximately 1to approximately 15; m can be an integer from approximately 6 toapproximately 100; n can be an integer from approximately 6 toapproximately 100; and Y can be a chelating agent. Preferably, the sumof m+n is at least 10. The chelating agent Y coating the one or moreworking electrodes to which a sample is in contact can be anaminocarboxylic acid; a hydrocarboxylic acid; ethelene diamine;diethylenetriamine; triethylenetetramine; triaminotriethylamine;polyethyleneimine; triethanolamine; n-hydroxyethylethylene diamine;2-aminopyridine; 4-aminopyridine; 2,2′ dipicolylamine; 5,6 diamino-1,10phenanthroline; thioglycolic acid; gluthathione; or diethyldithiophosphoric acid. In particular, the chelating agent Y can be anaminocarboxylic acid. The aminocarboxylic acid can be an iminodiaceticacid or n-hydroxyethyl glycine. The chelating agent Y can be ahydroxycarboxylic acid. The hydroxycarboxylic acid can be tartaric acid,citric acid, or gluconic acid. The method can further include measuringa first sampling current that flows through the one or more workingelectrodes during a predetermined interval in which the pulse voltage isnot applied, measuring a second sampling current that flows through theone or more working electrodes while the pulse voltage is applied, andcalculating the difference between the first and second samplingcurrents.

As described herein, the portable electrochemical tongue providesnumerous benefits. Providing a polymer coating on the working electrodeimproves the sensitivity and lowers the level of detection. Providing anelectrochemical tongue having more than one working electrode furtherimproves the ability to detect metal ions in complex mixtures, such asin groundwater analysis. Further, an electrochemical tongue providesimproved sensitivity in a device that is more portable than traditionalanalytical systems, such as atomic absorption spectroscopy andinductively coupled plasma mass spectrometry. Such in situ analysis canoffer significant cost savings because it permits on site detection in asingle step. Paired with a processor adapted for machine learning, sucha device can improve the speed and accuracy of on-site detection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a front view of one embodiment of a non-sampling electronictongue penetrometer.

FIG. 1B is a side cross section of the non-sampling electronic tonguepenetrometer of FIG. 1A.

FIG. 1C is a detail of a general schematic of down-hole potentiostatdevice component of the electrochemical tongue of FIGS. 1A and 1B.

FIG. 2A is a side view of another embodiment of a non-samplingelectronic tongue penetrometer.

FIG. 2B is a side cross-section of the non-sampling electronic tonguepenetrometer of FIG. 2A displaying detailed components extracted (fromleft to right: optional vibration dampening system, reference electrode,electrode housing, exterior mounting bracket, and working electrodes).

FIG. 3A is a front view of an embodiment of a sampling electronic tonguepenetrometer.

FIG. 3B is a cross-section of the sampling electronic tonguepenetrometer as represented in FIG. 3A.

FIG. 3C is a plan view in cross-section, of the electronic tonguepenetrometer as represented in FIGS. 2A and 2B.

FIG. 4 is block diagram for a microcontroller controlled potentiostatsystem suitable for use with an electronic tongue as disclosed herein.

FIG. 5 is a schematic for a potentiostat circuit suitable for use withan electronic tongue.

FIGS. 6A-6D are images of a virtual instrument (VI) for data extractionand management suitable for use with an electronic tongue.

FIG. 7 is a flowchart showing a system for detecting metal ions in acomplex mixture.

FIG. 8 is a plot of detection of Zn²⁺, Cd²⁺, Pb²⁺, and Hg²⁺ in aqueoussamples (at 5 ppm) using a prior art uncoated working electrode.

FIG. 9 is a plot of detection of Zn²⁺, Cd²⁺⁺, Pb²⁺, and Hg²⁺ in sand at5 mg/kg using a prior art uncoated working electrode.

FIG. 10 is a plot of differential pulse stripping voltammograms for Cd²⁺of various sub-ppm concentrations using one embodiment of a PTCPTAmodified glassy carbon electrode.

FIG. 11 is a plot of calibration curves for Cd²⁺ obtained using oneembodiment polythiophene copolymer modified glassy carbon electrode.

FIG. 12 is a series of differential pulse stripping voltammograms forvarious sub-ppm concentrations of Cu²⁺ using a poly(hydroxyl phenylacetic acid) modified glassy carbon electrode of one embodiment of theelectrochemical tongue of the invention in aqueous solutions.

FIG. 13 is a calibration curve for various sub-ppm concentrations ofCu²⁺ using a poly(hydroxyl phenyl acetic acid) modified glassy carbonelectrode of an embodiment of the electrochemical tongue of theinvention in aqueous solutions.

FIG. 14 is a plot of simultaneous detection of Cd²⁺, Pb²⁺, and Cu²⁺ inaqueous samples at 500 ppb using a poly(hydroxyl phenyl acetic acid)modified glassy carbon electrode of an embodiment of the electrochemicaltongue.

FIG. 15 is a plot of multi-electrode responses to Zn²⁺, Cd²⁺, Pb²⁺, andHg²⁺ in water employing an electrochemical tongue.

FIG. 16 is a differential pulse stripping voltammogram of Cd²⁺ atvarying concentrations using a PTAA-coated glassy carbon electrode of anelectrochemical tongue of the invention.

FIG. 17 is a calibration curve for Cd²⁺ using a PTAA-coated glassycarbon electrode of an electrochemical tongue of the invention.

FIG. 18 is a differential pulse stripping voltammogram of Pb²⁺ atvarying concentrations using a poly(HPA)-coated glassy carbon electrodeof an electrochemical tongue of the invention.

FIG. 19 is a calibration curve for Pb²⁺ using a poly(HPA)-coated glassycarbon electrode of an electrochemical tongue of the invention.

FIG. 20 is a differential pulse stripping voltammogram of Cd²⁺ atvarying concentrations using a poly(HPA)-coated glassy carbon electrodeof an electrochemical tongue of the invention.

FIG. 21 is a calibration curve for Cd²⁺ using a poly(HPA)-coated glassycarbon electrode of an electrochemical tongue of the invention.

FIG. 22 is a differential pulse stripping voltammogram of Cu²⁺ atvarying concentrations using a poly(HPA)-coated glassy carbon electrodeof an electrochemical tongue of the invention.

FIG. 23 is a calibration curve for Cu²⁺ using a poly(HPA)-coated glassycarbon electrode of an electrochemical tongue of the invention.

FIG. 24 is a comparison of the response to simultaneous detection of 500ppb of Cd²⁺, Pb²⁺, and Cu²⁺ employing a electrochemical tongue of theinvention.

FIG. 25 is the observed formal potential for sensor responses for Zn²⁺,Cd²⁺, and HG²+ at four different types of electrodes.

FIG. 26 is the peak areas for the four different types of electrodesrepresented in FIG. 25.

FIG. 27 is the peak currents for the four different types of electrodesrepresented in FIG. 25.

FIG. 28 is the peak slopes for the four different types of electrodesrepresented in FIG. 25.

FIG. 29 is a calibration plot for Pb²⁺ using an uncoated carbon fibermicroelectrode.

FIG. 30 is a calibration plot for Cd²⁺ using an uncoated carbon fibermicroelectrode.

FIG. 31 is a plot showing principal component analysis (PCA) employingan electrochemical tongue of the invention.

FIG. 32 is a decision tree model generated for classification.

FIG. 33 shows the response of a poly-HPA coated glassy carbon electrodeof the invention that has been overcoated with NAFION to 100 ppb Pb²⁺.

FIG. 34 is a differential pulse stripping voltammogram for differentconcentrations of Pb²⁺ in 100 mm 2-(N-morpholino)ethanesulfonic acid(MES) buffer (pH 6.5) employing an electrochemical tongue of theinvention.

FIG. 35 is a calibration curve for Pb²⁺ using the electrochemical tongueemployed to generate the differential pulse stripping voltammogram ofFIG. 34.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

Described herein is an exemplary electrochemical tongue having areference electrode, a counter electrode, and one or more workingelectrodes. A potentiostat can be in electrical communication with thereference electrode, counter electrode, and one or more workingelectrodes. The working electrode can be coated with a polymer havingthe formula (I):

The variable X can be NH or O. The variable r can be an integer fromapproximately 1 to approximately 15. The variable Y is a chelatingagent.

In one embodiment, X is oxygen, r is 1, and Y is a carboxylic acid,which yields a compound having the following structural formula (IV):

The phenol can be polymerized from phenol monomers using chemicalcatalysts, enzyme catalysts (such as Horseradish peroxidase (HRP)), orbiomimetic catalysts (such as Iron Salen). In biomimetic and enzymecatalyzed synthesis, the polymerization is initiated by the addition ofhydrogen peroxide, which also acts as an oxidant. The monomer can alsobe polymerized by electrochemical polymerization using cyclicvoltammetry. For example, the polymer can be electrochemicallypolymerized, yielding the following polymer coating on the electrode,where n can be an integer from approximately 6 to approximately 100:

The electrodes can be formed of diverse materials and have many suitablegeometries. The electrodes can be of gold, carbon fiber, silver,platinum, or transparent conductive oxides, or any other suitablematerial. The electrodes can be microelectrodes.

In a specific embodiment, the electrochemical tongue of the inventiondescribed herein can be inserted into a cone penetrometer, as anembodiment of the invention that is referred to as an “electronictongue” or simply an “E-tongue.” Inserting the electrochemical tongueinto a cone penetrometer can provide a device for rapid, real-timeanalysis of electro-active species in complex subsurface strata.

An exemplary E-tongue of the invention has an electrochemical tongue asdescribed herein integrated within a standard cone penetrometer. TheE-tongue can include non-sampling and/or sampling sensors. In anon-sampling E-tongue, the working and reference electrodes directly candirectly contact the sample (e.g., soil having water with heavy metalcontaminants). The non-sampling sensors are located on the conical tipor along the sleeve of the probe, exposed to the soil, wherevoltammetric analysis is performed for identification and quantificationof toxic metals at the electrode/soil interface. The electrodes for thenon-sampling tongue should be robust because they directly contact thesoil as the probe is pushed through the soil, and the friction canabrade the sensing surface of the electrode. The sampling sensorsinvolve a system that draws contaminated water into the cone body (andbrought in contact with the electrode array) under hydrostatic head orvacuum pressure across a porous filter.

Increased sensitivity, particularly for the detection of a plurality ofanalytes, can be improved by providing two or more working electrodes.While not all working electrodes need a conductive polymer coating,providing additional working electrodes with such a polymer coating canfurther improve sensitivity.

An exemplary E-tongue of the invention is illustrated in FIGS. 1A-1C.The cone penetrometer has a conical tip 11 and cylindrical tube 19. Anexemplary cone penetrometer is described is ASTM D3441-05. The conepenetrometer is typically 3.57 cm in diameter with a 60 degree conicaltip.

The cone penetrometer is designed to house various electrodearrangements and geometries. The welded steel section 14 extends theouter diameter to fit a multitude of sensing systems, additionallycreating a flat surface for mounting electrodes 24 and 17. Section 14 isnot necessary where sensors do not require a flat surface or additionalarea within the penetrometer. Section 14 includes a cylindricalextrusion to allow the sensor housing 12 to mount in position. Thesensor housing can be fabricated from steel with insulation positionedat connector pins 21 and lead wires 16, eliminating possible shortcircuiting, or made of non-conducting materials such as high-densitypolyethylene. The exterior mounting bracket 13 serves to fixate theelectrode housing 12 along the welded steel section 14. Preferably, themounting bracket 13 is approximately ⅛th inch thick and consists ofsteel or high strength non-conducting materials (e.g. polycarbonate).The cylindrical extrusions 25 can serve as openings for workingelectrodes 24 and a reference electrode 17. Alternatively, cylindricalextrusion 25 can include a porous membrane and/or salt bridge to protectelectrodes 17 and 24 (with electrodes recessed into the probe). Asdescribed herein, electrodes 24 can be coated with a polymer.

Optionally, a vibration dampening system 22 can be included tocompensate for external vibrations. The vibration dampening system 22can be a rubber mount or spring system that reduces resonant vibrationstransmitted throughout the cone shaft to the electrochemical sensorarray.

The reference electrode 17 is designed for stability, and should notcontain toxic materials (such as a saturated calomel electrode) toprevent possible in situ contamination. As shown in FIGS. 2A-2B, thereference is a Ag/AgCl electrode, with a leakless frit 20 exposed to thesoil. This electrode includes a 90 degree bend in the body to upholdstability while remaining compact in the space provided in thepenetrometer. The electrode housing 12 should provide a space 23 for thereference electrode to slide in and out with ease for the non-samplingprobe. A straight reference electrode can be used for the samplingprobe, as long as the conductive frit 20 is exposed to the sampledwater. The counter electrode can be any exposed conductive material,such as the penetrometer sleeve 19 for the non-sampling probe or thedouble threaded stud 26 for the sampling probe.

An exemplary sampling probe is illustrated in FIGS. 3A-3C. In a samplingprobe, the heavy metal contaminated fluid (e.g., from within the soil)is sampled through a porous filter into the interior of the device wherethe electrodes are located. In this configuration, the electrodes areless prone to damage because they are more protected. A porous ring 27allows an analyte, such as contaminated water, to flow into anelectrochemical tongue located within the probe body. The porous ring 27can be constructed of a porous metal alloy, stone, or plastic. In apreferred embodiment, the thickness of the porous ring is approximately⅛th inch. Contaminated water can flow across the porous boundary underhydrostatic pressure or suction (e.g., generated from vacuum,peristaltic, or diaphragm pumping systems) within the cone body. Adouble threaded stud 26 can hold the penetrometer body together whileholding the porous ring in place at the female threaded sections 29. Thethreaded stud 26 can be hollow to allow electrical wires to passthrough. Additionally, the threaded stud 26 can be used as a counterelectrode within the electrochemical tongue. One of skill in the artwill recognize, however, that the counter electrode need not bethreaded. One or more working electrodes 24 and a reference electrode 17can be housed in any location within the electrochemical tongue, exposedto the sampled groundwater, as shown in cross-section A-A of FIGS.3A-3C. The sensor housing 28 serves to hold electrodes in place, whileinsulating connector pins 21 and lead wires 16.

The sensing materials used for working electrodes 24 generally areessentially electrochemically inert over a wide range of potentials.These materials are not intended to optimize resulting current signalsnor achieve selectivity toward any one ion. Rather, the aim is toincorporate a sensor array of two or more electrodes with the choice ofelectrodes based on ruggedness (durability for field use), stability,and high cross-sensitivity.

Preferably, each sensor in the array produces a different response tothe same set of analytes. The combined response from the sensor arrayshould produce a stable integrated response representing the signaturefor the particular analyte that is sensed. These electrodes can befabricated in any way to provide a robust sensing surface (e.g.mechanically, micro-nano pipetting, ink jetting, or lithographically).

The potentiostat device 15 can be designed to fit within the cone body,adjacent to the electrodes. This configuration is referred to asdown-hole control. Alternatively, the potentiostat can be positioned ata location independent of, and far away from, the multi-sensor array,such as at the ground surface or next to a computer). The potentiostatsystem should preferably accommodate voltage waveforms and currentranges required for the incorporated electrode array. Exemplaryconstructions are provided in U.S. Pat. Nos. 8,152,977 and 4,426,621,the teachings of which are incorporated by reference in their entirety.

In another embodiment, the invention is a microcontroller-basedpotentiostat system capable of performing cyclic voltammetry analysis.FIG. 4 shows a block diagram of one embodiment of a potentiostat systemof the invention, which consists of a microcontroller, a potentiostatcircuit, and level shifters. A MSP430F1611 microcontroller (TexasInstruments, Dallas, Tex., United States), for example, can be used,which is an in-built digital-to-analog converter (D/A converter), analogto digital converter (A/D converter), and universal asynchronousreceiver (UART) buffer used for communication between themicrocontroller and computer. The microcontroller generates a triangularwaveform voltage using 12-bit D/A converter. Since the output voltage isunipolar, an op-amp level shifter circuit is used to make the outputvoltage bipolar. The level shifted voltage is applied to a potentiostatcircuit as shown in FIG. 5.

The potentiostat circuit includes three operational amplifiers (op amp).One (op-amp1) is used for a current buffer of which the current at theoutput is capable of providing infinite current. A second op amp(op-amp2) is used for a voltage follower. This results in a voltagedifference between the working and reference electrode to be identicalas the applied triangular input voltage, generated by the D/A converter.The last op amp (op-amp3) is employed for a current-to-voltageconverter, with output voltage proportional to the current between theworking and counter electrode. The output voltage corresponding to thecurrent is read by the A/D converter after bipolar output voltage isshifted into unipolar voltage using an additional level shifter.

In one embodiment, a virtual instrument (VI) for data extraction andmanagement employs LabVIEW programming. Two different types of anodicstripping voltammetry having a square and a differential pulse wave formwere implemented on the VI. As shown in FIGS. 6A-6D, the VI consists offour tab windows. The waveform parameters of square wave anddifferential pulse potentials, such as initial, height, increment, andfinal potentials, can be input on the first tab window (see FIG. 6A).The configuration parameters can be sent to the microcontroller usingRS232 communication by pushing a button of ‘PM_SET’. Once the MSP430microcontroller receives the parameters, it generates a sweepingpotential between the working electrode and a reference electrode andthe resulting current response at the working electrode is measured. Rawdata of applied potential and induced currents, including forward andreverse currents at each working electrode, are displayed and graphed onFIG. 6B and FIG. 6C, respectively. The voltammogram of the sensorresponses from the four working electrodes were also plotted on FIG. 6D.

In another embodiment using a database of sensor responses obtained bycontrolling voltage at the working electrode's surface and measuring theinduced current (generated by oxidation-reduction processes ofelectro-active species in solution), correlation of the in situ sensorresponses with the database by the method of the invention, andemploying an electrochemical tongue of the invention results in bothqualitative and quantitative evaluation of analytes in real-time.

FIG. 7 is a flowchart for an intelligent training and prediction modulethat can be implemented on a microcontroller unit (MCU). In step 100,the MCU receives a voltammetric response from the working electrode. Instep 200, the MCU filters the data and extracts features. The extractedfeatures can be the formal potentials and peak currents fromvoltammetric responses. For example, the MCU can filter the RAW datausing a moving average algorithm (as a smoothing technique) anddetermine features (peak current and its formal potential) using a peakdetection algorithm. The peak detection algorithm can identify peaks bytaking the first derivative of the raw sensor response. Among themaximum points, the final peak point can be determined by twouser-defined threshold values in terms of the slope of derivative andmagnitude of sensor response. Step 300 pertains to a training mode,having substeps 320 and 340. In step 320, a decision tree model is builtusing the extracted formal potentials. The decision tree has a specificstructure (e.g., numbers of nodes and branches). In an initial state ofa decision tree, a root node is the first node to which the training setare assigned. If the training sets at the root node consist of two ormore classes, a test node is made that will split the training set intotwo subspaces, or secondary nodes. These can either become terminalnodes, in which a classification is reached, or another test node. Theprocess is repeated until each branch results in a terminal node and acompletely discriminating tree is obtained. An exemplary built tree isillustrated in FIG. 34, where terminal nodes represent the labeled heavymetal ions. In step 240, a linear model is built using a Least SquareEstimator (LSE), which creates a linear model using the training data(e.g., x-axis: concentrations; y-axis: peak currents). Thus, the MCUcan: i) smooth and filter data; ii) identify formal potentials andmeasure peak currents (or other relevant parameters); and iii) build adecision tree learning and linear model. To reduce noisy signals fromraw sensor responses, a moving average algorithm (as a smoothingtechnique) has been programmed into the MCU. The MCU can also implementa testing mode 400, in which metal ions can be detected in a complexsample. The decision tree and linear model from the training mode 300are used to predict the identity and concentration of the metal ions.Since the trained decision tree and linear model have criticalparameters (e.g., slope and intercept of linear models), the unseen datacan determine the identity and concentration of metal ions.

EXEMPLIFICATION

The following are examples of representative embodiments of theinvention and are not intended to be limiting in any way.

Example 1 Prior Art

This example shows the detection of zinc, cadmium, lead, and mercury inwater using a multi-sensor array of uncoated microelectrodes. A solutioncontaining 5 ppm of each of Zn²⁺, Cd²⁺, Pb²⁺, and Hg²⁺ was prepared inwater. The solution was tested using an array of microelectrodes fittedon a cone penetrometer. Gold, silver, and carbon fiber basedmicroelectrodes were used in this test. As illustrated in FIG. 8, thegold based microelectrode showed good sensitivity towards Pb²⁺ while theresponse to Zn²⁺ and Hg²⁺ was negligible. The carbon fiber electrode andsilver showed good sensitivity to the metal ions in solution.

Example 2 Prior Art

This example shows detection of Zn²⁺, cd²⁺, Pb²⁺ and Hg²⁺ in sand usinga multi-sensor array of microelectrodes. A solution containing 5 mg/kgof each of Zn²⁺, Cd²⁺, Pb²⁺ and Hg²⁺ was prepared in saturated sand. Thesolution was tested using an array of microelectrodes fitted on a conepenetrometer. Gold, silver and carbon fiber based microelectrodes wereused in this test. As illustrated in FIG. 9, the gold basedmicroelectrode showed poor sensitivity towards the metal ions in sand,while the carbon fiber electrode and silver showed good sensitivity tothe metal ions. However, in case of the silver microelectrode, prior tothe oxidation of mercury a large increase in the back current wasobserved.

Example 3

This example shows detection of Cd²⁺ in water using a glassy carbonelectrode (GCE) having a thiophene-based copolymer coating of oneembodiment of the electronic tongue of the invention. The electrode wascoated with poly(thiophene-co-n-Pyridin-4-yl-2-thiophen-3-yl-acetamide)(“PTCPTA”) having the following structural formula (V):

In structural formula (V), m can be an integer from approximately 6 toapproximately 100, and n can be an integer from approximately 6 toapproximately 100. Preferably, the sum of m+n is at least 10.

FIG. 10 shows the differential pulse stripping voltammograms of varioussub-ppm concentrations of Cd²⁺ in acetate buffer (100 mM, pH 4.5).Results indicate a detection limit of 30 ppb, which is improved level ofsensitivity compared to the non-coated membrane. FIG. 11 is acorresponding calibration curve for Cd²⁺ obtained using thepolythiophene copolymer modified electrode of the invention.

Example 4

The example shows detection of Cu²⁺ in water using a polyphenol-modifiedglassy carbon electrode of an electrochemical tongue of one embodimentof the invention. The electrode was coated with polymer having thefollowing structural formula, where n can be an integer fromapproximately 6 to approximately 100:

The poly(hydroxyl phenyl acetic acid) modified glassy carbon electrodewas used for the detection of Cu²⁺ in aqueous solutions. Thedifferential pulse stripping voltammograms (DPSV) for differentconcentrations of Cu²⁺ in a 50 mM phosphate buffer are shown below inFIG. 12. The poly(hydroxyl phenyl acetic acid) modified GCE is capableof detecting concentrations as low as 100 ppb of Cu²⁺. A calibrationcurve was also prepared, shown in FIG. 13.

Example 5

This example shows simultaneous detection of Cd²⁺, Pb²⁺, and Cu²⁺ inwater using a poly(hydroxyl phenyl acetic acid) of another embodiment ofa modified glassy carbon electrode (GCE) of an electronic tongue of theinvention. A solution containing 500 ppb of Cd²⁺, Pb²⁺, and Cu²⁺ in 50mM phosphate buffer was prepared and tested using the poly(hydroxylphenyl acetic acid) modified GCE. As illustrated in FIG. 14, the polymermodified electrode can detect all three metals with good sensitivity.The sensor demonstrated greater sensitivity to Cu²⁺ than the other metalions.

Example 6

This example illustrates electrode sensitivity toward a multitude oftarget metal ions. FIG. 15 illustrates the variation in sensor responseto an aqueous solution containing Zn²⁺, Cd²⁺, Pb²⁺, Pb²⁺, and Hg²⁺. InFIG. 15, AUME refers to gold microelectrode; CFME refers to carbon fibermicroelectrode; AGME refers to silver microelectrode; PTME refers toplatinum microelectrode. While CFME displayed well-defined peak currentsdue to the presence of all four target ions, Pb²⁺ was primarily dominantat the AUME. Additionally, the oxidation of solid metal substrates ofthe PTME and AGME hindered responses from highly cathodic/anodic ions(i.e., Zn²⁺ and Hg²⁺, respectively) yielding a narrow potential windowfor detecting Pb²⁺ and Cd²⁺. The solid metal electrodes (AUME, PTME, andAGME) were also found to be selective towards dissolved oxygen, whichwas highly valuable for determining influences of DO levels on peakcurrent responses of target ions.

Example 7

This example illustrates an electrode coated with poly(thiophene aceticacid) (PTAA). FIG. 16 is a DPSV for Cd²⁺ of varying concentrations usinga PTAA modified GCE, and FIG. 17 is a calibration curve for Cd²⁺ usingthe PTAA modified electrode. As FIG. 16 illustrates, the polymer coatedelectrode demonstrated a level of detection in the parts-per-billionrange.

Example 8

This example demonstrates an electrode coated with a polyphenol. Thephenol monomer is 4-hydroxyphenylacetic acid (HPA), which has thefollowing structure (IV):

The phenolic monomer was electrochemically polymerized on a GCE bypotential cycling (CV) a 2.5 mM solution of monomer dissolved in 50 mMperchloric acid between −0.7 and 1.25 V for 100 cycles.

All tests were performed in 5 ml solutions of metal salts (lead nitrate,cadmium chloride, and copper (II) chloride) prepared in phosphate buffer(pH 6.5, buffer strength 50 mM). DPSV techniques were performedsubsequent to metal ion deposition at −1.5 V (vs. Ag/AgCl) for 300seconds. Differential pulse parameters used include a 75 mV pulseheight, 100 ms pulse width, and a 750 ms period with a 6 mV stepincrement. Each pulse was swept between −1.5 and 0.1 V vs. Ag/AgCl.

FIG. 18 shows the DPSV for different concentrations of Pb²⁺, and FIG. 19shows a corresponding calibration curve. As shown in FIG. 18, thepoly(HPA) coated GCE can detect concentrations down to 100 ppb of Pb²⁺.

FIG. 20 shows the DPSV for different concentrations of Cd²⁺, and FIG. 21shows a corresponding calibration curve. As shown in FIG. 20, thepoly(HPA) coated GCE can detect concentrations down to 50 ppb of Cd²⁺.

FIG. 22 shows the DPSV for different concentrations of Cu²⁺, and FIG. 23shows a corresponding calibration curve. As shown in FIG. 22, thepoly(HPA) coated GCE can detect concentrations down to 100 ppb of Cu²⁺.

The response of the poly(HPA) coated GCE was compared to a clean GCE toconfirm the improvement in the level of detection of Pb²⁺, Cd²⁺, andCu²⁺, as shown in Table 1.

TABLE 1 Metal Ion Peak Current Response on (100 ppb Peak CurrentResponse on poly(HPA) modified clean concentration) Clean GCE (μA) GCE(μA) Lead No response 0.265 Cadmium 0.345 1.495 Copper No response 0.705

The poly(HPA) coated GCE was tested in a complex solution containingPb²⁺, Cd²⁺, and Cu²⁺ and compared the an uncoated GCE in the samesolution. As show in FIG. 24, the poly(HPA) modified shows superiorsensitivity for Cu²⁺.

Example 9

This example demonstrates the response generated by four differentmicroelectrodes for four metal ions (in separate solutions), each atfive concentrations, with a total of five iterations for each.Parameters extracted to establish quantitative and qualitative analysisinclude peak currents, peak areas, slope of responses induced throughthe onset of oxidation, and observed formal potentials. FIGS. 25-28 showthe results of these parameters for 10 ppm Zn²⁺, Cd²⁺, Pb²⁺, and Hg²⁺.No responses to Zn²⁺ were observed at the PTME due to high back currents(inherent in the formation of hydronium ions during electrolysis)evident at highly cathodic potentials, which masked responses from thetarget ion.

Error bars in FIGS. 25-28 display the variability in sensor responsesover the five iterations. The magnitudes of peak currents were dominantfor both Cd²⁺ and Pb²⁺, but greater ranges in responses were alsoevident. This was attributed to the low ionic strength of solutions, aswell as the close proximity (considering Formal Potentials) to DO. Tofurther outline variance between iterations, two calibration plots (peakcurrents versus target ion concentration) are shown in FIGS. 29 and 30for Pb²⁺ and Cd²⁺, respectively, at a CFME.

To overcome the variability between iterations, machine learningtechniques, such as are known in the art, were implemented. BothPrincipal Component Analysis (PCA) and a decision tree were used toanalyze the results, as shown in FIGS. 31 and 32, respectively. PCAresults showed well separated clusters of each heavy metal and 99.8% ofthe sample variance was captured by the first two principal components.Well separated clusters in the Principal Component Analysis indicatesthat the system can be trained to accurately classify ions. If theclusters in the PCA plot overlap, another type of electrode could beused to more clearly differentiate the overlapping clusters. Thedecision tree can classify the ions accurately using responses from justthe PTME and CFME electrodes, though more can be used.

Example 10

This example demonstrates that the robustness of the polymer film can beimproved by over-coating. Chemically synthesized poly(hydroxyl phenylacetic acid) was dissolved in a solution of ammonium hydroxide.Solutions of varying concentration of polymer were prepared. 10 μL ofthe polymer solution was drop cast on the surface of a glassy carbonelectrode. The solvent was then evaporated in order to obtain a castfilm of the polymer. Approximately 2 μL of a Nation solution in methanolwas cast onto the surface of the polymer modified electrode to improvethe robustness of the coating. The response of the polymer coatedelectrodes containing varying polymer concentration to 100 ppb Pb²⁺ wascompared in order identify an optimal concentration of the polymerneeded to improve the sensitivity of the sensor. A comparison of thesensor response of the polymer coated electrodes of varying coatingthickness to 100 ppb of Pb²⁺ is shown in the FIG. 33. While this exampledemonstrates overcoating with NAFION, one of ordinary skill in the artwill understand that the overcoating can be any suitable conductivefluoropolymer.

Example 11

This example demonstrates detection of trace concentration of Pb²⁺ usingpoly(HPA) modified electrodes. For each concentration tested, a newpolymer film was cast. The differential pulse stripping voltammograms(DPSV) for different concentrations of Pb²⁺ in a 100 mM MES buffer (pH6.5) are shown in FIG. 34, and a calibration curve is shown in FIG. 35.The poly(HPA) modified glassy carbon electrode is capable of detectingconcentrations as low as 10 ppb of lead (lower than the maximumcontamination limit) recommended by the US EPA.

The teachings of all patents, published applications and referencescited herein are incorporated by reference in their entirety.

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. An electrochemical tongue, comprising: a) areference electrode; b) a counter electrode; c) one or more workingelectrodes, wherein at least one of the one or more working electrodesis coated with a polymer or copolymer having the formula:

wherein X is independently NH or O; r is independently an integer fromapproximately 1 to approximately 15; and Y is a chelating agent; and d)a potentiostat in electrical communication with the reference electrode,the counter electrode, and the one or more working electrodes.
 2. Theelectrochemical tongue of claim 1, wherein the electrochemical tonguehas at least two working electrodes that are of distinct materials. 3.The electrochemical tongue of claim 1, wherein at least one of the oneor more working electrodes are formed from gold, carbon fiber, silver,platinum, and transparent conductive oxides.
 4. The electrochemicaltongue of claim 1, wherein at least one of the one or more workingelectrodes are of glassy carbon, carbon paste, carbon fiber, carbonnanotubes, and graphene.
 5. The electrochemical tongue of claim 1,wherein at least one of the one or more working electrodes comprisesconductive metal oxides coated on rigid or flexible substrates.
 6. Theelectrochemical tongue of claim 2, wherein at least one of the one ormore working electrodes is coated with a polymer having the formula:

wherein X is NH or O; r is an integer from approximately 1 toapproximately 15; n is an integer from approximately 6 to approximately100; and Y is a chelating agent.
 7. The electrochemical tongue of claim2, wherein at least one of the at least two working electrodes is coatedwith a polymer having the formula:

wherein X is NH or O; r is an integer from approximately 1 toapproximately 15; m is an integer from approximately 6 to approximately100; n is an integer from approximately 6 to approximately 100; and Y isa chelating agent.
 8. The electrochemical tongue of claim 1, wherein thechelating agent Y is selected from the group consisting of anaminocarboxylic acid; a hydrocarboxylic acid; ethelene diamine;diethylenetriamine; triethylenetetramine; triaminotriethylamine;polyethyleneimine; triethanolamine; n-hydroxyethylethylene diamine;2-aminopyridine; 4-aminopyridine; 2,2′ dipicolylamine; 5,6 diamino-1,10phenanthroline; thioglycolic acid; gluthathione; and diethyldithiophosphoric acid.
 9. The electrochemical tongue of claim 8, whereinthe chelating agent Y is an aminocarboxylic acid.
 10. Theelectrochemical tongue of claim 9 wherein the aminocarboxylic acid isselected from the group consisting of iminodiacetic acid andn-hydroxyethyl glycine.
 11. The electrochemical tongue of claim 8,wherein the chelating agent Y is a hydroxycarboxylic acid.
 12. Theelectrochemical tongue of claim 11, wherein the hydroxycarboxylic acidis selected from the group consisting of tartaric acid, citric acid, andgluconic acid.
 13. The electrochemical tongue of claim 1, furtherincluding a sensor selected from the group consisting of a voltammetricsensor, an amperometric sensor, and a potentiometric sensor.
 14. Theelectrochemical tongue of claim 13, wherein the sensor includes one ormore of a linear sweep sensor, a cyclic sensor, a stair case sensor, adifferential pulse sensor, a square wave sensor, and an anodic/cathodicstripping voltammetry sensor.
 15. The electrochemical tongue of claim13, further including a potentiometric sensor.
 16. The electrochemicaltongue of claim 13, further including one or more of a redox sensor, apH sensor, an electrical conductivity/sensitivity sensor, a dissolvedoxygen sensor, and a selective ion selective sensor.
 17. Theelectrochemical tongue of claim 1, further including a housing defininga passageway for the reference electrode, counter electrode, and one ormore working electrodes.
 18. The electrochemical tongue of claim 17,wherein the housing is adapted for insertion into a penetrometer. 19.The electrochemical tongue of claim 17, further including a porousportion that allows a sample to enter the electrochemical tongue. 20.The electrochemical tongue of claim 1, wherein at least a portion of atleast one of the one or more working electrodes include sensing surfacesthat have been modified by at least one of gel integration andbismuth/mercury coating.
 21. The electrochemical tongue of claim 1,wherein the one or more of the working electrodes has an ionicallyconductive fluoropolymer overcoating.
 22. The electrochemical tongue ofclaim 1, further comprising a processor programmed to: i) receive avoltammetric response; ii) filter and extract features; iii) build adecision tree and a linear model; and iv) display the identity of ametal ion.
 23. A method for performing voltammetry, comprising the stepsof: a) contacting a sample to be analyzed with an electrochemical tonguethat includes: i) a reference electrode; ii) a counter electrode; iii)one or more working electrodes, wherein at least one of the one or moreworking electrodes is coated with a polymer or copolymer having theformula:

wherein X is NH or O; r is an integer from approximately 1 toapproximately 15; and Y is a chelating agent; and iv) a potentiostat inelectrical communication with the reference electrode, the counterelectrode, and the one or more working electrodes; b) applying aconstant voltage across the one or more of the working electrodes toreduce the metal ion onto the surface of the electrode; and c)increasing the voltage across one or more working electrode to oxidizeand strip off the metal from the surface of the electrode.
 24. Themethod of claim 23, wherein the electrochemical tongue has at least twoworking electrodes of distinct materials to which the sample iscontacted.
 25. The method of claim 24, wherein at least one of the oneor more working electrodes are formed from gold, carbon fiber, silver,platinum, and transparent conductive oxides.
 26. The method of claim 24,wherein at least one of the one or more working electrodes to which asample is contacted includes at least one carbon-based material selectedfrom the group consisting of glassy carbon; carbon paste; carbon fiber;carbon nanotubes; and graphene.
 27. The method of claim 24, wherein atleast one of the working electrodes to which a sample is contactedcomprises conductive metal oxides coated on rigid or flexiblesubstrates.
 28. The method of claim 24, wherein at least one of the atleast two working electrodes to which a sample is contacted is coatedwith a polymer having the formula:

wherein X is NH or O; r is an integer from approximately 1 toapproximately 15; n is an integer from approximately 6 to approximately100; and Y is a chelating agent.
 29. The method of claim 23, wherein atleast one of the at least two working electrodes to which a sample iscontacted is coated with a polymer having the formula:

wherein X is NH or O, r is an integer from approximately 1 toapproximately 15; m is an integer from approximately 6 to approximately100; n is an integer from approximately 6 to approximately 100; and Y isa chelating agent.
 30. The method of claim 23, wherein the chelatingagent Y coating the one or more working electrodes to which a sample isin contact is selected from the group consisting of an aminocarboxylicacid; a hydrocarboxylic acid; ethelene diamine; diethylenetriamine;triethylenetetramine; triaminotriethylamine; polyethyleneimine;triethanolamine; n-hydroxyethylethylene diamine; 2-aminopyridine;4-aminopyridine; 2,2′ dipicolylamine; 5,6 diamino-1,10 phenanthroline;thioglycolic acid; gluthathione; and diethyl dithiophosphoric acid. 31.The method of claim 30, wherein the chelating agent Y coating the one ormore working electrodes to which a sample is in contact is anaminocarboxylic acid.
 32. The method of claim 31 wherein theaminocarboxylic acid is selected from the group consisting ofiminodiacetic acid and n-hydroxyethyl glycine.
 33. The method of claim30, wherein the chelating agent Y coating the one or more workingelectrodes to which a sample is in contact is a hydroxycarboxylic acid.34. The method of claim 33, wherein the hydroxycarboxylic acid isselected from the group consisting of tartaric acid, citric acid, andgluconic acid.
 35. The method of claim 23, further comprising the stepsof: d) measuring a first sampling current that flows through the one ormore working electrodes during a predetermined interval in which thepulse voltage is not applied; e) measuring a second sampling currentthat flows through the one or more working electrodes while the pulsevoltage is applied; and f) calculating the difference between the firstand second sampling currents.
 36. A method for performing voltammetry,comprising the steps of: a) contacting a sample to be analyzed with anelectrochemical tongue that includes: i) a reference electrode; ii) acounter electrode; iii) one or more working electrodes, wherein at leastone of the one or more working electrodes is coated with a polymer orcopolymer having the formula:

wherein X is NH or O; r is an integer from approximately 1 toapproximately 15; and Y is a chelating agent; and iv) a potentiostat inelectrical communication with the reference electrode, the counterelectrode, and the one or more working electrodes; b) ramping theworking electrode voltage linearly versus time to either positive ornegative voltages.